83 Nautique 2001 - Mercruiser Chev 350 260 issue

The pistons being in backwards will cause no issues at all. Baitkiller is correct for a reverse rotation application. FYI: Drag racers for years have reversed the pistons in some stock class racing because it makes a little less drag which can help at very high RPM.
Not good for a Pass Car engine but helps above 4,000 RPM.
Boats have a continuous supply of cold cooling water so I would not hesitate to take your block to .060 over if needed.

Need some help again.

Mechanic has gone to 40 thou and there is still scoring in the block. he is reluctant to goto 60 thou citing reasons that it can cause overheating etc in motors that are under a lot of stresS?

Could anyone confirm or is it safe to goto 60 on these old blocks?

What I see in your picture of the pistons is a clear case of 4 corner scuff.
Indicated by the black marks on the skirt face next to your piston pins.
A hot burn condition, usually caused by a lean mixture or bad timing setting heats the piston heads. The heat radiates to the Piston Pin boss. When the boss heats up it will cause the piston skirt to grow in the areas surrounding the piston pin boss.   When these areas grow and the engine is still running they start dragging on the cylinder wall.
Notice this dragging caused piston skirt scuffing only at the 4 corners and not in the center of the skirts.   This scuffing is black and silver, the black indicates oil was present when this overheat happened and burned the oil into your skirts.
If this was caused by a engine water temp overheat the pistons would scuff but they would scuff at the center of the skirts not at the 4 corners.
You had excessive combustion temperature which heats the piston head not water temp overheat.
Causes could be Lean Burn, normally caused by running with a vacuum leak or a lousy timing setting causing excessive combustion temp.
If you roll your pistons upside down and look at the bottom of the pistons heads you will see color change from the overheat..   
You will need new pistons, The bore will have to be honed again to remove the scuff marks, if they are severe it may need to be bored to the next oversize.
New Rings and a gasket set should get you running again.
Look closely at the upper halfs of your rod bearings. They may show damage from the pounding. When cylinder temps go this high it normally starts the engine detonating which can beat the crud out of your rod bearings.
Better check the heads to be sure they did not warp or crack. This combustion temp was very hot to have this happen.
If only one piston looked like this we would have another discussion but with them all looking the same it is very clear what happened.
Note: Find what caused this or you may have the exact same issue with the next engine you install.
I am sorry this happened and hope you can fix it for not much money.

Gary S wrote: What kind of raw water pump do you have? I have heard of the Sherwoods cam tearing up the impeller when they get worn but have never seen one do it. They do recommend changing it on a major rebuild.

Less than a year ago I replaced what was on their with a new Mercruiser pump housing and impeller. The old housing was cracked and impeller pretty shot.

One thing I don't have is a raw water filter/strainer in the boat, considering that maybe I should fit one and that may also help with initial possible run dry when first starting the boat as my boat sits out water a fair amount. Not sure if that will even help

8122pbrainard wrote: geecee wrote: First issue is the impeller failed, this has to have been a full failure as my temperatures have been fine and the impeller was quite new. virtually no fins left on the impeller. The unknown is what came first? Did the impeller cause the overheat or did something else cause the loss of water causing the impeller to fail?

I guess so, but as soon as the temp spiked I killed the motor, initially thought it was a loose wire on the temp sensor, after fiddling with the wire I concluded that it wasn't that and that the motor did seem a little warm.

Sat for a bit and randomly heard a loud almost suction/vacuum sound, kinda like hitting the bottom of a milkshake. Tried to start it and it didn't turn over, pulled plugs and water came pouring out,

geecee wrote: First issue is the impeller failed, this has to have been a full failure as my temperatures have been fine and the impeller was quite new. virtually no fins left on the impeller.

The unknown is what came first? Did the impeller cause the overheat or did something else cause the loss of water causing the impeller to fail?

Hercules_ZA wrote: geecee wrote: Havent had time to even look at the boat this week, but I may have found another 2001 in great condition (from the photos) all original even with the horns on the bow, has the pcm ford engine instead of the mercruiser chevy power plant. hoping to strike a deal next week and see where it goes. better half is a little concerned about having three boats in the yard however Hey Geecee Where in SA are you based?? I have a cracking mechanic who is passionate about the old classics, and doesn't break the bank either. Cheers d

Im hiding in Durban so hard to find decent guys here

Havent had time to even look at the boat this week,
but I may have found another 2001 in great condition (from the photos)
all original even with the horns on the bow, has the pcm ford engine instead of the mercruiser chevy power plant.

hoping to strike a deal next week and see where it goes.

better half is a little concerned about having three boats in the yard however

MrMcD wrote: I still have a scar on my forearm from a Jet Boat with open Dry Stack headers.   Had a carb flooding event on the river 30 years ago in a fast current, busy fixing the carb when a cabin cruiser went by and threw a 3 foot wave at us. The boat rocked and my forearm hit the exhaust.    I NEVER want a boat with dry stacks.   Took about a 2x2 piece of skin and burned it about 1/8 inch deep.   Took most of the summer to heal. Cool exhaust to the touch is a really nice upgrade. I don't care if it costs me some horse power. BTW: we switched that Jet to through the transom exhaust soon after the mishap. I don't think it cost 100 RPM wide open but made it much more pleasant to own. PS: Another cabin cruiser stopped and helped us, he even gave me Gin FIzzes to dull the burn pain. We fixed the carb and went back to skiing. Pretty sure that would not have happened without the Gin influence. Dealt with the burn later. Boaters can be amazing people at times!

Ouch! So many burns, so many tales--and that's just from cars. Yeah I'd never lift the cover if I knew there were glowing hot headers down there. I wonder what wins--aloe Vera or gin fizzies! I'll take the gin. I met Mike Ditka on a plane once and he said if there's something hurting you real bad, just go back out and play--because the next hurt will make you forget how badly the last one hurt.

The troubles I read about resulting from fresh water cooling of our exhaust manifolds is almost never-ending. The whole deal makes me wish for cherry red exhaust headers but I suppose there's going to be trouble getting that exhaust out of the boat without creating a burn hazard of some kind to the boat or the occupants!

I'm sure you know the conversion for ft-lb to n-m but figured I'd mention it anyways

first increment 25 ft=lb equals 33.9 n=m

second increment 45 ft-lb equals 61 n-m

Final torque 65 ft-lb equals 88.1 n-m

I know somebody named me who has looked at the wrong scale on the torque wrench before and been quite low on torque values so I figured I'd just throw that out there too    Thought I was going to 75 ft -lb but really went to 75 n-m which worked out to about 55 ft-lb. Needless to say, I had a little extra torquing to do when I figured that out.

In the photo above and not seen is a ball check valve which allowed me to close off the hose I used to pressurize the system. Once I had it at 30 PSI in the photo I closed the valve and walked away. After an hour when it was still holding pressure I sprayed all the fittings and connections with a mix of dish soap and water. I found a couple of very slow leaks at the connections, The soap and water will bubble where the leaks are externally. I also sprayed around the intake manifold ends where I know the coolant passages are. When I had no more external leaks I pumped back up to the full 30 PSI sprayed everything again and left it for 2 days. I was still holding pressure. so I know the heads, intake manifold, head gaskets and block are sealed. Then we fixed his exhaust manifold,

MrMcD wrote: You already pulled the engine apart so some tests can't be done now but you can burn a ton of oil because of bad valve stem seals. Have your mechanic inspect your valve stem seals and the valves. Your block could be good for more miles if the heads are fixed. A company called World Products sells brand new heads complete for your engine and they are not that expensive in the USA. They might be able to ship you a set in S Africa.

Engine is still together currently.
Valves stem seals were replaceed when head was done.

What tests can you recommend?

Also if anyone knows the best way I can get this water out without stripping the engine or doing further damage

I'm am really concerned about getting someone local to rebuild this engine.
Right now I am considering a crate motor but it will break the bank for me here.

I may as well cut my losses and finance a newer boat

next option as mentioned is picking up a reman base engine from the states and transferring the exterior items from mine across to the new powerplant.

I always suspected a dodgy ring, but with full compression I thought I was in the clear.
Now with the excessive smoking i figured it must have a bad ring or two over and above the obvious valve issues I had, I don't want to throw more money at the topend only to find the bottom has issues, so looks like the engine definitely needs to be pulled.